1. Field of the Invention
This invention relates to a ferroelectric liquid crystal material. More particularly it relates to a ferroelectric liquid crystal composition comprising smectic liquid crystal compound(s) and optically active compound(s) and having high-speed response properties and a light switching element using the same.
2. Description of the Related Art
Liquid crystal compounds have been broadly used as a material for liquid crystal elements, but most of such liquid crystal display elements are of TN display mode, and as liquid crystal materials, those belonging to nematic phase have been used.
Since TN display mode is non-emissive, it has specific features that eyes are not tired and power consumption is small, but on the other hand, it has drawbacks that the response is slow and the display is not seen depending on the angle of view. Thus, the mode has recently been being turned toward a direction of making the best use of its characteristics as flat display, and in particular, high-speed response properties and a broad angle of view have been required.
Various improvements in liquid crystal materials in response to such requirements have been attempted. However, as compared with other emissive displays such as electroluminescence display, plasma display, etc., TN display mode is considerably inferior in the aspect of the response time and the breadth of angle of view.
In order to make the best use of the specific features of liquid crystal display elements such as non-emissive mode and low power consumption and also secure response properties matching emissive display, development of a novel liquid crystal display mode in place of TN display mode has been indispensable. As one of such attempts, a display mode utilizing the light switching phenomenon of ferroelectric liquid crystals has been proposed by N. A. Clark et al (see Appl. Phys. Lett., vol. 36, p. 899 (1980)).
The presence of ferroelectric liquid crystals has been announced by R. B. Meyer et al for the first time (see J. Phys. vol. 36, p. 69 (1975)), and the phases of the crystals include chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase and chiral smectic H phase (hereinafter abbreviated to Sc* phase, S.sub.I * phase, S.sub.F * phase, S.sup.*.sub.G phase and S.sup.*.sub.H phase, respectively in the aspect of liquid crystal structure.
Various specific features are required for ferroelectric liquid crystal materials used for practically usable ferroelectric liquid crystal display elements, but at present, there is no single compound which satisfies all the specific features; hence it is necessary to use ferroelectric liquid crystal compositions obtained by mixing some liquid crystal compounds or liquid crystal compounds with non-liquid-crystal compounds.
Further, the ferroelectric liquid crystal compositions are not limited to those consisting only of ferroelectric liquid crystal compounds, but it has been reported that using compound(s) or composition(s) each exhibiting achiral smectic C, F, G, H, I phase or the like phase (hereinafter abbreviated to Sc phase or the like) as base substance(s), at least one compound exhibiting ferroelectric liquid crystal phases is mixed with the above compound(s) or composition(s) to constitute a ferroelectric liquid crystal composition as a whole (see Japanese patent application laid-open No. Sho 61-195187/1986). Further, it has also been reported that using compound(s) or composition(s) each exhibiting S.sub.c phase or the like as base substance(s), at least one compound which is optically active but exhibits no ferroelectric liquid crystal phase is mixed with the above compound(s) or composition(s) to constitute a ferroelectric liquid crystal composition as a whole (see Mol. Cryst. Liq. Cryst., 89, 327 (1982)).
In a brief summary of these facts, it is seen that when at least one optically active compound, irrespective of whether or not the compound exhibits ferroelectric liquid crystal phase, is mixed with a base compound, it is possible to constitute a ferroelectric liquid crystal composition.
A smectic liquid crystal mixture comprising the above-mentioned base substance exhibiting at least one of achiral S.sub.c phase or the like and having at least one of S.sub.c phase or the like will hereinafter referred to as "smectic base mixture".
As such smectic base mixture, liquid crystal mixtures exhibiting S.sub.c phase within a broad temperature range including room temperature are practically preferred. As a component of the smectic base mixture, some compounds selected from among liquid crystal compounds of e.g. phenylbenzols, Schiff's bases, phenylpyrimidines, 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, etc. have been used.
For example, Japanese patent application laid-open No. Sho 61-291679/1986 and PCT International publication No. WO 86/06401 disclose a ferroelectric liquid crystal obtained by mixing a 5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compound, and the former publication discloses that use of a ferroelectric smectic liquid crystal material using the above pyrimidine derivative as a smectic base mixture makes it possible to shorten the response time of light switching elements. Further the above former publication also discloses that a ferroelectric liquid crystal material consisting of a 5-alkyl-2-(4'-alkylbiphenylyl-4)pyrimidine, the above 5-alkyl-2-(4-alkyloxyphenyl)pyrimidine and an optically active compound is also effective for improving response properties.
However, as compared with liquid crystal displays of other modes such as emissive display, a further improvement in the response properties has been desired for the above liquid crystal display.
On the other hand, one more problem desired to solve, together with the improvement in the response properties is to reduce the temperature-dependency of the response time. Current ferroelectric liquid crystal materials have a large temperature-dependency of the response time so that a cross talk phenomenon often occurs due to environmental temperature change to notably reduce the display quality of the display.
Thus, a ferroelectric liquid crystal material having a small temperature-dependency of the response time together with high-speed response properties has been earnestly desired.